Abstract

In 1989, a deep test in Cottle County, Texas, discovered an anomalously thick, gas-charged Pennsylvanian (lower Bend Group) clastic section along the Matador arch. Subsequent exploration and development provided data that support the concept that the natural gas fields in Cottle and King counties, north-central Texas, mark the extent of a hydrocarbon system related to the Broken Bone graben, an elongate 180 km2 pull-apart basin in southeastern Cottle County. The graben results from left-step overstepping of left-lateral fault zones and is a component of the Red River-Matador structural trend of the greater Ancestral Rocky Mountains. Arkosic detritus originating from the Amarillo-Wichita uplift was transported southward, over the region containing the graben, toward the Knox-Baylor trough. Episodic graben subsidence accommodated a part of this sediment load as syntectonic, cyclically stacked Bend Group (Atokan, lower Pennsylvanian) fluvial-deltaic to marine deposits. Organic facies within the graben fill are predominantly terrestrially derived (gas prone) and present in sufficient quantity for significant hydrocarbon generation. Lopatin method basin modeling, vitrinite reflectance (Ro) measurements, and Ro-calibrated pyrolysis-derived maturity measures demonstrate that the Bend Group organic facies in the graben have approached peak gas-generating maturation levels. Generated gas migrated within and outside of the basin following nonsealing faults and channelized fluvial pathways into several reservoir rock types in combination structural and stratigraphic traps.